1. Technical Field
This invention pertains to phase shifters, i.e., electronic components capable of altering the phase of an electronic signal.
2. Description of the Prior Art
A phase shifter device is depicted in FIG. 1. Phase shifter 10 includes input port 11 for receiving an input signal, output port 12 for providing an output signal, and control port 13 which receives a control signal which controls the phase of the output signal relative to the input signal. It is often desired to make this phase shift between the output and input signals continuously variable over a wide range, and to allow a wide instantaneous bandwidth. Such phase shifters can thus be used for wideband signals, for example, a QAM digital radio signal.
One embodiment of a prior art phase shifter is shown in FIG. 2. Phase shifter 20 includes circulator 14, which is a three port device. As shown in FIG. 2, the input signal received on input port 11 is provided to varactor diode 15 by circulator 14. The resulting signal reflected off varactor diode 15 is applied to output port 12, and serves as the output signal. Phase control lead 13 receives a DC (or modulating) control signal which is applied to the anode of varactor diode 15, whose cathode is connected to ground. This phase control signal controls the capacitance of varactor diode 15, and thus the phase shift between the output signal on output port 12 and the input signal on input port 11.
One disadvantage of prior art phase shifter 20 of FIG. 2 is that circulators are practical only at microwave frequencies because of the size and magnetic field requirements of ferrite circulators. Furthermore, with a prior art phase shifter as in FIG. 2, delay times change with phase shift, preventing constant group delay.
Another embodiment of a prior art phase shifter is shown in the schematic diagram of FIG. 3. Phase shifter 13 includes quadrature (90.degree. ) splitter 16. The input signal is split into two output signals on leads 21 and 22 that reflect back into splitter 16 and emerge from output port 12. Ninety degree phase splitter 12 isolates and impedance matches input port 11 and output port 12. Varactor diodes 17 and 18 are connected between output ports 21 and 22, respectively, of splitter 16, and ground. Varactor diodes 17 and 18 receive a phase control signal from lead 13 which changes their capacitance, and thus the phase shift of the output signal on output port 12. However, quadrature splitters are themselves difficult to design and have poor amplitude and phase matching as compared with in-phase splitters. For example, quadrature splitters measured across a one octave frequency band typically provide output signals within .+-.3.degree. from quadrature and having 1 dB amplitude balance. 180.degree. hybrid splitters measured across a two decade frequency band typically provide output signals within .+-.3.degree. and have an amplitude balance of approximately 0.3 db. In contrast, in-phase hybrid phase splitters measured across a two decade frequency band typically provide output signals within .+-.1.degree. and having amplitude balance of approximately 0.2 db.
Furthermore, VHF quadrature splitters are typically limited to a bandwidth of only one octave. Phase shifters made with quadrature splitters thus require two separate reactive networks, in this case varactor diodes 17 and 18. These reactive networks must be made identical in order to preserve the theoretical input and output port matches at leads 21 and 22.
One notable disadvantage of prior art devices which utilize a single varactor diode capacitor to obtain a variable phase shift is the inability to provide a constant group delay. The diode capacitance is set to give a desired phase shift at one particular frequency. When the signal being phase shifted is more complex than a simple sinusoid, the group delay of the phase shifter over the signal bandwidth is also important. Group delay is the negative of the derivative of the phase with respect to frequency, i.e. the negative of the slope of a phase versus frequency plot. A constant delay (slope) is required in order to prevent the phase shifter from distorting the output signal. An example of such distortion is the introduction of I to Q channel crosstalk on a digital modulation signal. The delay of a single varactor diode reactive network varies as the phase shift is changed. This is apparent from a plot of phase versus frequency for this type of network, as shown in FIG. 4.
For a phase shift of P.sub.1 degrees at frequency W.sub.1, the capacitance is set to value C.sub.1. For a phase shift of P.sub.2 degrees at the same frequency W.sub.1, the capacitance is set to value C.sub.2. The plot shows that the group delay (slope of the plots) for frequencies around W.sub.1 varies as the phase shift is changed.
A prior art example of a circuit using a 180.degree. hybrid splitter in a variable reactance network for use in IF stages of FM microwave radio links is described by Shiki et al., "IF Variable Equalizers for FM Microwave Radio Links", IEEE Transactions on Circuits and Systems, Vol. CIS-21, No. 4, July 1974, pages 517-527. Shiki et al. describe an equalizer for providing desired shapes of the curve depicting relative time delay versus frequency. Thus, Shiki et al. provide an equalizer circuit for equalizing the group delay of preceding circuit elements so as to provide a desired overall group delay curve, for example an overall group delay which is constant. However, Shiki et al. do not teach or even suggest a phase shifter which itself has a constant group delay.